Streamflow Characteristics and Trends in New Jersey, Water Years 1897 - 2017

Science Center Objects

Water resource managers regularly utilize streamflow statistics from the USGS for planning and regulation across the state. The impacts of climate, water use, and land use changes can alter streamflow over time. Periodically updated streamflow statistics are essential for the effective management of water resources in New Jersey.

Stream gage staff in the Bear Brook

Staff Gage in the Bear Brook at Park Ridge, New Jersey

(Public domain.)



Streamflow statistics from USGS gages are necessary for water resource planning and for programs to regulate activities across New Jersey.  Streamflow statistics are used to establish surface-water withdrawal limits and to plan for future water supply needs.  These statistics are also used for waste load allocations and to analyze the impacts of non-point source discharges to streams.  In addition, the variability of flow (low, high and peak) and flow trends can alter riparian habitat which can affect stream ecosystems.  Since streamflow can be influenced by both human activities and climate changes, it is important for watershed planning and regulation, to understand the reasons for trends in flow in order to develop and implement good management strategies.


The USGS New Jersey Water Science Center last published a report on Streamflow Characteristics and Trends in New Jersey in 2005 (Watson and others, 2005, USGS Science Investigative Report SIR 2005-5105).  The report summarized streamflow statistics at gaging stations for the period 1897-2001, and at partial record stations through 2003. Since that time streamflows may have changed in response to changes in water use, land development, and climate (Figure 1).  Because these statistics are used for water resource planning and for regulatory activities across the state, periodic updates are needed. 





Project Objective

Graph of Recorded Streamflow

Period of record daily stream flow at USGS Streamgaging Station 01463620 – Assunpink Creek near Clarksville, NJ. A possible change in low flows appears to have occurred after the break in the record. Mercer Lake dam was built upstream in 1975. The dam, water withdrawals, and climate all likely influence flows at this gage.

(Public domain.)


This project will ensure NJDEP has the most updated information for water supply planning and waste load allocation decisions.  It will also assist the NJDEP and other agencies in evaluating watershed plans for the 20 watershed management areas, and in assessing if streamflows are changing in HUC11 basins identified as having limited availability in the State Water Supply Plan. The proposal addresses priority issues of the cooperative program by providing information that will help the NJDEP address water quality and quantity issues.


The vast amount of streamflow information collected through 2017 will be summarized with high and low flow frequency, duration, and annual descriptive statistics. Tests for possible trends in streamflow at both continuous-record streamgages and partial record sites will also be performed. Previous work (Watson and others, 2005) summarized trends at continuous record gages only. Baseflow separation of hydrographs will be performed on all continuous gages so that a long-term average of the sustained groundwater contribution of streamflow can be documented, as well as examined for trends.  The baseflow values as well as a subset of frequency statistics will be transferred to partial record sites via regression. A geodatabase will be produced with all basins delineated to organize and make available the massive amounts of information that will be produced. The geodatabase will include stats from Watson and others, 2005 so that comparisons to past statistical analysis can be easily made. The project team intends to investigate web-based tools for geographic display of statistics and trends for users to explore the results and customize their data needs.  The methods and results of the project will be summarized in a USGS Scientific Investigations Report (SIR).





Map of New Jersey Stream gage sites

Map of New Jersey showing the location of potential stream gages for this study; continuous-record stations (triangles) and partial record sites (circles).

(Public domain.)

Stream gages included in analysis (Map):

  • Continuous-record stations -  97 sites. 
  • Partial-record stations – a subset of 871 sites, of which 476 sites were used by Watson and others, 2005.


What Streamflow Statistics will be calculated?

  • Winter and summer n-day series and frequencies:
    • High flow – 1, 7, 30 –day series at 2, 5, 10, and 25-year recurrence intervals
    • Low flow 1, 7, 30 –day series at 2, 5, 10, and 20-year recurrence intervals
  • Duration tables
  • Period of Record (POR) statistics
    • Maximum, minimum, mean annual flow
  • Annual statistics
    • Annual peak to 3-day mean ratio
    • 25-75 flow duration ratio
  • Trend tests of n-day series and annual statistics
  • Annual variability in the form of a non-parametric coefficient of variation (IQR/Median; Watson and others, 2005 used parametric CV.)
  • Annual baseflow (for trend analysis) and POR mean baseflow
  • Trend tests of residuals of partial record site regressions
  • Trends with consideration of Long Term Persistence and Short Term Persistence (Hodgkins, 2017)





All streamflow data used to calculate streamflow statistics are publicly available on USGS NWISWeb.





Amy McHugh - Lead Hydrologist

Kara Watson - Hydrologist

Samantha Sullivan - Statistician